The present invention relates to underground conduits, and specifically relates to multiple pipes used as conduits wherein the pipes are installed together without an outer casing.
Transmission cables of various sorts are ubiquitous throughout the world's developed countries. Such cables include electrical power transmission lines as well as various types of communications cables, such as co-axial cable and fiber optic cable. When installed above ground, these cables will typically be carried along poles or towers in order to avoid contact between the cables and persons or objects at ground level. Above-ground installations are often not practical, however, in crowded urban environments. Above-ground installations also increase the risk that the cables will be damaged or destroyed, such as by falling trees or other objects, or by high winds or other weather-related causes. Finally, above-ground installations are generally considered unsightly, and thus are disfavored by the public. For these reasons, underground cable installation is generally displacing above-ground placement, particularly in urban and suburban areas.
Another option for cable placement, applicable where the obstacle to be overcome is a body of water, is submarine cabling. A submarine cable is simply laid on the floor of the body of water, often with an extra protective coating over the cable to protect from impacts or abrasions. Submarine cables are commonly used in ocean crossings and in connections between offshore platforms and the mainland. Submarine cables may not, however, be practical where the body of water to be crossed has a significant mono-directional current, as in the case of a swift river. Also, submarine cabling may not be practical when a crossing is required at a location that includes existing submarine cabling, pipes, or other such obstacles. Finally, submarine cabling may not be practical where the characteristics of the cables themselves, such as the magnetic and electric fields produced by the cables, may interfere with passing vessels or other submarine cables, pipes, or installations.
When a cable is to be laid across land and above-ground installations are found to be impractical, the alternative approach is underground cabling. The usual method of installing underground cables is to dig a trench in which the cable will be laid. The cable is often installed within a conduit in order to protect the cable from damage through contact with water or soil. Installation of cable conduit using the trenching method is relatively straightforward, but may be difficult or impossible in certain applications. Environmental issues may prevent the use of trenching, such as when a cable is to cross an environmentally-protected area such as a wetland or a beach. There are also circumstances in which trenching is simply not feasible, such as when a cable is to be installed under a body of water such as a river or lake. Trenching is also impractical, or at least prohibitively expensive, in some highly urbanized areas. If a cable is to be installed in an underground manner in these circumstances, an alternative to trenching must be considered.
The past decade has seen significant advances in the field of Horizontal Directional Drilling (HDD). HDD is a “trenchless” technique that allows for the construction of a relatively long underground tunnel through which a conduit may be pulled for an underground cable installation. Modern HDD equipment allows the user to construct a tunnel that, within certain limits, may twist and turn in order to avoid obstacles and place conduit through a desired path. This technique is thus ideal to pass conduits under rivers, lakes, highways, and the like, and is also employed on occasion to avoid trenching in highly-developed urban areas.
Although there are alternative “trenchless” methods for passing cables over waterways, such as the placement of conduits on the river bed or passing conduits over bridges that cross the waterway, HDD offers a number of advantages over these techniques. First, HDD allows a tunnel to be placed with great precision since the location and direction of the drilling head may be monitored and adjusted as the drilling operation is underway. HDD also allows the conduit to be placed at a sufficient depth that avoidance of other utility lines and other artificial obstacles is assured. Since HDD allows placement of conduit underground, the possible obstruction of a waterway or the possibility of damage to the conduit from passing watercraft is eliminated. Finally, since only the termination points on the HDD-installed cable are above ground, the technique requires only a relatively small amount of surface land for implementation, which may result in cost efficiencies as well as a lessening of the environmental impact of the construction.
A typical HDD process involves three main stages. The first stage is the drilling of a pilot hole, which in the case of a riverbed installation will pass from one bank of the river to the other. Often, an area must be built up or cleared in order to provide for the termination points at each river bank. The second stage is pilot hole enlargement, during which cutting heads are employed to enlarge the diameter of the wellbore pilot hole so that it will be sufficient to receive the desired conduit. Where significant enlargement is required, successively larger cutters may be employed in sequential fashion. A fluid is typically directed over the boring tool and back up through the wellbore in order to remove dirt and any other cut materials from the space. The final step is pullback installation of the conduit itself. In this step, the drilling pipe, which has reached the opposite bank from which it started, has the conduit attached in order to pull the conduit back through the wellbore. It may be noted that the wellbore is typically filled with a drilling mud or similar material in order to prevent collapsing until the conduit is in place.
In drilling the pilot hole, a boring system is situated on the ground surface and drills a hole into the ground at an oblique angle with respect to the surface. After the boring tool reaches a desired depth, such as beneath a riverbed, the tool may then be directed along a substantially horizontal path to create a horizontal wellbore. After the desired length of wellbore has been obtained, the tool is then directed upwards to break through to the earth's surface on the opposite bank. Thus the angle that the drilling head forms with the ground surface often starts out relatively severe, then flattens for a period as the obstacle, such as a river, is avoided, then rises more severely near the exit.
In a simple application such as the passing of a small conduit under a roadbed, the pilot hole may be sufficiently large that no enlargement step is required. In applications involving large conduits or multiple conduits, however, it will be necessary to use successively larger cutters in order to create a wellbore of the proper diameter. Alternatively, a reamer or swab may be employed in addition to the cutter, such that the reamer is attached to the drill string and is pulled back through the wellbore, thus reaming out the wellbore to a larger diameter, or smoothing the wellbore to make the desired diameter more regular within the wellbore. To place the conduit, it may be attached to the drill pipe after hole enlargement so that it is dragged through the wellbore and returned to the drilling rig.
A special problem related to large-scale HDD projects is the use of multiple conduits presented through the same wellbore. Installation of multiple conduits using a trenching approach presents little problem, as the conduits may simply be laid side-by-side, or alternatively in layers, within the trench. The size of the trench is thus adjusted to ensure that the conduits may be accommodated along with the desired spacing between conduits, as appropriate for the type of cables to be passed within the conduits. Since HDD techniques require pulling of conduit through the drilled tunnel, however, the use of multiple conduits create additional engineering problems due to the stresses and strains that result from the pulling of multiple conduits simultaneously.
HDD techniques have been applied to certain types of installations of multiple conduits through a single tunnel passing underneath rivers and waterways. These techniques have been applied where relatively small conduits are to be placed, such as a series of four-inch inside diameter (ID) high-density polyethylene (HDPE) conduits for carrying fiber optic communications cables. The pipes may simply be attached to the HDD drill rig and drawn simultaneously through the tunnel for placement. Because of the small size of the conduits used, no special problems are encountered for conduit placement in these conduit clusters that are not encountered when placing a single conduit.
The application of HDD technology to the installation of multiple larger conduits, however, presents special problems that have not been addressed in applications involving groups of smaller conduits. As pipe sizes become larger, simply pulling the pipes in a group back through the wellbore will not be possible because of the drag created as the pipes are pulled through the wellbore. This is particularly true when HDPE pipes are used. It should be noted that metal pipes cannot be used in certain applications because conducting materials such as metals will interfere with the operation of communications signals passing through co-axial cables. Metals also may not be used in certain applications involving the laying of electrical transmission and distribution lines.
The drag imparted upon conduits during placement in a wellbore are of two varieties: fluidic drag, and friction between the conduit or conduit group and the wellbore wall. Fluidic drag results from the fill material that is pumped into the wellbore during pipe placement; this material typically being a bentonite-based mud. The fluidic drag created depends upon the viscosity and density of the mud, the geometry of the pipe or pipe group, the Reynolds number of the conduit material, and the speed at which the conduit group is pulled. Since the drag on the conduit is proportional to the square of the speed at which the conduit is pulled, pull speed is an important factor in limiting fluidic drag. The friction between the conduit and the soil or wellbore wall depends upon the normal force acting on the group and the coefficient of friction of the wellbore wall.
One potential solution to the problem of multiple large pipes pulled through a wellbore is to add an outer pipe or casing. This solution has been commonly applied to applications such as undersea pipelines and offshore oil rig supply cables. The use of a casing serves to protect the inner pipes and cables from damage, and prevents them from becoming entangled as may occur if they were laid separately. European patent application no. 785,387 to Koninklijke PTT Nederland N.V. teaches an example of a method of installing a group of communications cable conduits within an outer casing.
While the use of a strong outer casing would solve some of the problems related to the installation of a group of larger pipes within a wellbore, the use of such a casing would significantly increase the cost of the installation project. Not only would the cost of the casing itself be a factor, but the use of a casing would require multiple cable pulls in order to complete the project. The casing would be laid within the wellbore during a first pull, then the conduit pipes themselves would be pulled through the casing within the wellbore. Given the enormous cost of such a project, it is critical to avoid any unnecessary expense, and it would thus be highly desirable to develop a method of installing a group of larger conduit pipes within a wellbore without the use of an outer casing.
In addition to the problem of cost, the use of an outer casing is also impractical for certain applications. For example, in applications that involve the placement of high-voltage transmission lines, heat dissipation is a critically important factor in the design of a conduit system. The application of an outer casing would reduce the ability of the system to dissipate heat from the high-voltage cabling to the surrounding earth, thereby requiring a larger conduit for the cable than would otherwise be required. The larger conduit would increase the cost of the installation, and might make installation impractical once conduit size reached beyond a certain practical limit. Also, it is believed that it would be necessary to use a metal casing in applications of this size since other available materials would not exhibit sufficient strength to serve as the outer casing. The use of a metal outer casing would create magnetic/electrical conduction problems when used to encase a high-voltage transmission line, thus again making the use of a large outer casing impractical for this type of application.
The limitations of the prior art are overcome by the present invention as described below.